Move truthy-function docs from “optional checks” to “enabled by default”

This error was enabled by default since its introduction (#13686);
document it in the correct section.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Author: andersk

docs/source/error_code_list.rst

@@ -764,6 +764,21 @@ the provided type.
 
    assert_type([1], list[str])  # Error
 
+Check that function isn't used in boolean context [truthy-function]
+-------------------------------------------------------------------
+
+Functions will always evaluate to true in boolean contexts.
+
+.. code-block:: python
+
+    def f():
+        ...
+
+    if f:  # Error: Function "Callable[[], Any]" could always be true in boolean context  [truthy-function]
+        pass
+
+.. _ignore-without-code:
+
 Report syntax errors [syntax]
 -----------------------------
 

docs/source/error_code_list2.rst

@@ -256,21 +256,6 @@ what the author might have intended. Of course it's possible that ``transform``
 and so there is no error in practice. In such case, it is recommended to annotate ``items: Collection[int]``.
 
 
-Check that function isn't used in boolean context [truthy-function]
--------------------------------------------------------------------
-
-Functions will always evaluate to true in boolean contexts.
-
-.. code-block:: python
-
-    def f():
-        ...
-
-    if f:  # Error: Function "Callable[[], Any]" could always be true in boolean context  [truthy-function]
-        pass
-
-.. _ignore-without-code:
-
 Check that ``# type: ignore`` include an error code [ignore-without-code]
 -------------------------------------------------------------------------